The present invention relates to an electron microscope that measures, inspects or observes a specimen by use of an electron beam, and particularly, relates to a scanning electron microscope provided with a function to make an axis adjustment of an electron beam, or a scanning electron microscope suitable for measurement of a height or charge-up of a specimen.
Size reduction and high integration of semiconductor devices have been rapidly advanced, and length measurement and inspection techniques have increasingly become more important these days. Scanning electron microscopes are an apparatus that observes a surface of a specimen by scanning the specimen with a focused electron beam and detecting secondary electrons or reflected electrons. Providing high resolution, the scanning electron microscopes are widely used as semiconductor length measurement and inspection apparatuses which are represented by a CD-SEM (Critical Dimension-Scanning Electron Microscope), a DR-SEM (Defect Review-Scanning Electron Microscope), or the like.
In order to observe the specimen with high resolution using such an apparatus, the conditions for the apparatus need to be adjusted as appropriate. For example, when a trajectory of an electron beam is deviated from the center of an objective lens, aberration is generated and thus image quality deteriorates. To avoid this, optical axis adjustment needs to be made before observation. Resolutions of respective apparatuses vary due to a difference among the apparatuses, i.e., a so-called machine difference, which poses a problem in improving measurement repeatability between the apparatuses. The conventional techniques for diagnosing and adjusting the apparatus conditions include following methods.
Patent document 1 discloses a method in which: a particular point such as an end of a knife edge or a center point of a cross mark is firstly measured at multiple focus levels; and the position of an objective lens aperture is automatically adjusted so that the specific points measured at the respective focus levels overlap each other.
Patent document 2 proposes a charged particle beam device which makes a focus evaluation or a focus adjustment before changing the deflecting conditions of an alignment deflector; or which is provided with a table of an amount of the focus adjustment corresponding to the deflecting conditions of the alignment deflector, and makes the focus adjustment in accordance with the table after changing the deflection conditions of the alignment deflector.
The above-mentioned semiconductor inspection and measurement apparatuses are designed to be installed in a semiconductor production line and to operate without human intervention. Moreover, for a speedup in inspection and length measurement, it is necessary to reduce each processing time and to capture a clear image at a high speed without focus deviation.
Generally, a focus adjustment is made by using a method of performing an automatic focus adjustment on the basis of SEM images captured at different focus planes. However, the method requires time, and thus throughput decreases. In order to reduce the time required for a focus adjustment, Patent document 3 discloses a method of focusing according to a wafer surface height by detecting the surface height with an optical height detector. Furthermore, patent document 4 discloses a method of making an adjustment to correct focus deviation caused by charge-up. In this method, the charge-up voltage is measured with an electrostatic potentiometer before actual measurement, and a focus adjustment is made on the basis of the measured voltage and the height of the specimen measured with an optical height detector.
In addition, patent document 5 discloses a technique in which: scores of images obtained by using different beam energies are analyzed; and the focus adjustment is made by adjusting the beam energy in accordance with this analysis.
Patent document 1: Japanese Patent Application Publication No. 2005-276639
Patent document 2: Japanese Patent Application Publication No. 2007-141632 (corresponding to US2007/0120065)
Patent document 3: Japanese Patent Application Publication No. 11-149895 (corresponding to U.S. Pat. No. 6,107,637)
Patent document 4: Japanese Patent Application Publication No. 2005-338096 (corresponding to U.S. Pat. No. 6,946,656)
Patent document 5: Japanese Patent Application Publication No. 2001-236915 (corresponding to U.S. Pat. No. 6,521,891)